On the mechanism of ligand binding to myoglobin. The role of structural fluctuations.

The association reaction of CO and O2 with heme is expected to reflect the differences in the electronic structures of the two ligands. CO binding should be controlled by a high spin/low spin transition while oxygen binding is spin-allowed. Dioxygen should thus bind substantially faster than CO. The experimental association rates of the two ligands are, however, almost identical. We propose that the reaction is triggered in both cases by a fast structural intermediate which allows the CO molecule to bind adiabatically. A suitable structural transition has been identified recently by inelastic neutron scattering.